In a Wireless Fidelity (WiFi for short) system based on an IEEE 802.11 protocol, a protocol stack of a radio interface includes a Medium Access Control (MAC for short) layer protocol. To reduce an inter frame space, channel contention time, and repeated transmission of a physical header when data transmission is performed by using the MAC layer protocol, 802.11n proposes a MAC layer aggregate transmission mechanism, referred to as an Aggregate MAC Protocol Data Unit (A-MPDU for short). In the MAC layer aggregate transmission mechanism, one physical layer convergence procedure Protocol Data Unit (PPDU for short) using an A-MPDU structure may include at least one MAC Protocol Data Unit (MPDU for short), where all MPDUs included in the PPDU are sent to a same receiving device.
As stipulated in an existing standard, when a receiving device receives a PPDU using an A-MPDU structure, the receiving device needs to reply to a transmitting device with a block acknowledgement (BA for short) frame, where the BA frame includes a block acknowledgement bitmap (BA Bitmap) field, and each bit of the BA Bitmap field corresponds to one of all MPDUs whose receiving statuses need to be fed back.
Because a maximum of 64 MPDUs can be aggregated in a PPDU using an A-MPDU structure, it is stipulated in the existing standard that a length of a BA Bitmap field is always 64 bits. However, in actual application, a quantity of MPDUs aggregated in the PPDU using the A-MPDU structure is far less than 64. In this case, if the length of the BA Bitmap field is still 64 bits, BA Bitmap field redundancy may be caused, resulting in resource waste.